1. Field of the Invention
The invention relates in general to a rake finger receiver and method thereof, and more particularly to a rake finger receiver and method therefor in a spread spectrum communication system.
2. Description of the Related Art
In a spread spectrum communication system, such as CDMA (Code Division Multiplexing Access) system, a mobile station requests a unique pseudo-random code (PN), called spreading code, from a base station for transmitting data. Every mobile station generates a spread signal for data transmission according to a unique spreading code. The receiver of the base station despreads the received spread signal from the mobile station according to the corresponding spreading code of the mobile station. However, there is a to-be-solved issue, multipath. Multipath is the propagation phenomenon that results in radio signals' reaching the receiving antenna by two or more paths, and the received signal from one of the paths is called a multipath component. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from terrestrial objects. The terrestrial objects can be mountains or buildings, for example. The arrival time of each multipath component is different. FIG. 1 is the time delay power spectrum of the multipath components received by the base station. Multipath components P1, P2, and P3 represent the same information bit, and the different arrival time of each multipath component is due to the propagation delay. The arrival times of each of the multipath components P1, P2, and P3 are t1, t2, and t3, respectively.
In a wireless spread spectrum communication system, a rake receiver is a popular receiver architecture for use in multipath propagation environment. The rake receiver demodulates the wireless signals received by different antennas and outputs the information bits originated from the respective mobile stations. Generally, the rake receiver includes multiple rake fingers and combiners. Each rake finger receives a multipath component, and produces a demodulated signal by adjusting the time delay of the received multipath component and then despreading the adjusted multipath component. The combiner combines the despreaded signals that belong to the same radio link and are outputted by the rake fingers using, for example, the maximum ratio combining technique, in order to obtain the maximum signal to noise ratio (S/N ratio), and thus the bit error rate is reduced. Two estimation techniques are used to obtain the characteristics of the channel. One is to obtain the time delay information of each multipath component for accurately despreading the received signal. The other is to obtain the channelweights, such as the relative amplitude and phase information among the multipath components.